An Integrated Approach to Manage Ascochyta Blight in Chickpea

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Sustainable Crop Protection

Results from the Pesticide Risk Reduction Program

Ascochyta blight, caused by the pathogenic fungus Ascochyta rabiei, is the most important challenge limiting chickpea production in western Canada. The disease occurs in nearly all chickpea growing areas. The fungus attacks all above-ground plant parts and infection can occur on leaves, stems and pods at any stage of plant growth (Fig. 1A-C), but plants are most susceptible to disease during flowering. Ascochyta blight causes low yields with poor seed quality and can result in up to 100% yield loss, especially in susceptible varieties (Fig. 1d).

Description of this image follows.

Fig. 1. Symptoms of ascochyta blight disease on (A) leaves, (B), stems and (C) pods of chickpea plants and (D) total crop failure caused from severe disease.

To investigate and develop integrated control practices, the Pest Management Centre's Pesticide Risk Reduction Program funded a study carried out by Agriculture and Agri-Food Canada scientists in collaboration with researchers at University of Saskatchewan and pulse industry representatives in the Swift Current region of Saskatchewan. In this study, commercial chickpea varieties and breeding lines were evaluated for resistance to ascochyta blight, pathogen isolates were screened for resistance to currently used fungicides and the effect of canopy manipulation (through altering seeding pattern, selection of variety type and varying plant density) on disease control was assessed.

The findings of this study, some of which are presented here, used in combination with other agronomic recommendations may help chickpea growers in making more informed cropping and crop protection decisions.

I. How to prevent disease before it strikes?

Use disease-free and treated seeds

Using certified, disease-free seed is key to a successful ascochyta blight management program. Treating seeds with a carbathiin- and thiabendazole-based product (e.g., Crown) will help minimize seed-borne disease in both kabuli and desi varieties. An additional seed treatment with metalaxyl-based product (e.g., Apron) is recommended for kabuli varieties which are susceptible to seed rot diseases. Producers should be aware that Saskatchewan Crop Insurance requires seeds with ascochyta-infection at or below 0.3% for coverage. Also, planting machinery needs to be equipped with a seed-feeding mechanism capable of handling large sized seeds to avoid physical damage. Damaged seeds are highly vulnerable to infection by soil-borne pathogens.

It is essential that Rhizobium inoculants are compatible with the fungicides and that the fungicide is applied first and dried before Rhizobium inoculants are to be applied. Use of granular inoculants instead of seed-applied inoculants will increase Rhizobium viability.

Select fields with no or less ascochyta history

Chickpea should be grown in an area with a non-host crop surrounding it. New plantings should be at least 500 meters away from fields that had chickpea crops the previous season, especially if the crop might have been infected.

Rotate chickpea every three to four years

In western Canada, ascochyta pathogen can be persistent on infected crop resides for 3 to 4 years. Thus, a 3- to 4-year rotation cycle with non-host crops is required between successive chickpea crops. The pathogen may be present in the field even four years after the initial chickpea crop, but the amount of fungal inoculum declines substantially after the first two intervening crops. However, in areas where the infection is through long distance dispersal of pathogen via winds, crop rotation may be of limited benefit.

Select less susceptible varieties

Chickpea varieties differ in their susceptibility to ascochyta blight. Under similar disease pressure, varieties with pinnate (fern-like) leaves (i.e. Amit, CDC Chico, CDC Frontier, CDC Yuma) show consistently lower susceptibility to ascochyta blight than varieties with unifoliate leaves (i.e. CDC Diva, Sanford, CDC Xena) at all growth stages (Fig. 2a). Similarly, desi types (all with pinnate leaves) are less susceptible to ascochyta blight than kabuli types (Fig. 2b).

Planting less susceptible (also known as partially resistant) varieties can improve control efficiency by disrupting the infection cycles of the disease during a growing season.

Description of this image follows.

Fig. 2. Severity of ascochyta blight in relation to (A) leaf type (pinnate vs. unifoliate), and (B) variety type (kabuli vs desi). Data shown are averaged across 3 years and 2 sites.

Description - Figure 2

Progress and severity of ascochyta blight disease during a growing season and at all chickpea growth stages (seedling, early flowering, late flowering and late pod) in relation to (A) chickpea varieties with different leaf types (for example fern-like vs. unifoliate leaf forms), and (B) chickpea variety type (kabuli vs desi). Varieties with fern leaves and desi types show consistently lower susceptibility to disease than varieties with unifoliate leaves and kabuli types. Data shown are averaged across 3 years and 2 sites.

Severity of ascochyta blight in the different crop growth stages. The following data is approximate.

  Seeding Early flower Late flower Late pod
Unifoliate leaf type 21% 30% 39% 49%
Fern leaf type 7% 9% 18% 16%
Kabuli variety 16% 21% 29% 32%
Desi variety 5% 8% 18% 16%

For more detailed information on recent recommendations of variety choice, refer to the 'Varieties of Grain Crops 2009' published by Saskatchewan Ministry of Agriculture (also available on-line; the web address is listed at the end of this document).

Aim for optimum plant density

Optimum density for best yield is 38 to 44 plants/m2 for kabuli chickpea and 44 to 50 plants/m2 for desi chickpea in western Canada. Typically, plant density within the recommended range does not appear to have an association with the severity of ascochyta blight when less susceptible varieties are used.

It is crucial to establish an optimum plant density to achieve best seed yield in chickpea, because seed yield is linearly associated with plant density (Fig. 3). Under high disease conditions, as plant density increased from 20 to 70 plants/m2, disease severity was shown to increase in highly susceptible varieties (e.g. CDC Xena, Evans), but remains constant or even decrease in the least susceptible varieties (e.g. CDC Cabri). Once ascochyta blight develops in a field, susceptible varieties quickly become infected, regardless of plant density.

Description of this image follows.

Fig. 3. Chickpea seed yield in response to plant density under the same disease pressure.

Description - Figure 3

Variations in seed yield (ranging from 1,000 - 5,000 kg/ha) in chickpea varieties CDC Anna, CDC Cabri, CDC Xena, and Evans in response to increasing plant density (20, 30, 40, 50, 60, 70, and 80 plants/m2) and under the same disease pressure. Seed yield is linearly associated with plant density and disease severity was shown to increase with increasing as plant density in highly susceptible varieties.

Plant population (plants m-2) Seed yield kg ha-1 for cultivar Evans Seed yield kg ha-1 for cultivar CDC Xena Seed yield kg ha-1 for cultivar CDC Cabri Seed yield kg ha-1 for cultivar CDC Anna
20 900      
30 1100 2100 3100 3200
40 1500 2500 3200 3800
50 1700 2500 3600 4000
60 1800 2450 3800 4100
70   2450 3500 4300

Consider paired-row seeding as an alternative planting configuration

Changing planting patterns from solid rows (i.e., spaced at 20-25 cm between rows) to a paired-row planting (i.e., 25-cm intra-row, and 75-cm between the paired-rows) (Fig. 4A) has shown to decrease ascochyta blight severity in the crop by an average of 16%, reduce use of fungicides by up to 30%, while maintaining or increasing yield by as much as 30% in highly-susceptible varieties. These results were achieved by using a modified sprayer system equipped with 2 arms and 3 angled nozzles, one on the top and one other nozzle on each side of the plant canopy (Fig. 4b).

The 3-nozzle sprayer provides a uniform coverage and delivers fungicide droplets directly into the lower part of the crop canopy where it is needed most (Fig. 4c). In conventional solid row planting, about 30-50% of the fungicide droplets are wasted on the bare ground when canopy is open early in the season, and later, when the canopy is closed, fungicide droplets are mainly trapped on the top of the canopy. Some modifications on the drill and sprayer are required in order to implement the paired-row planting.

Description of this image follows.

Fig. 4. Chickpea seeded in paired-rows (A); a schematic 3-nozzle sprayer system (B); and optimal coverage of foliage by fungicide droplets (C).

II. What to do when the disease strikes?

Know your foe and check regularly for its presence in the field

Regular scouting and monitoring is the key for early disease detection and timely management actions. Refer to the 'Scouting and Management of Ascochyta Blight in Chickpea' guide, published by the Saskatchewan Ministry of Agriculture (available only as a booklet) for tips on how to recognize the pathogen and disease symptoms, understand disease development, determine disease risk and make sound decisions about judicious use of fungicides in the field.

Rotate fungicide chemistries

Currently available fungicides for use in chickpea include chlorothalonil, azoxystrobin, pyraclostrobin, boscalid and prothioconazole. These belong to different chemistry groups with varying modes of action (except for azoxystrobin and pyraclostrobin, which are both in the same group). Fungicides should be rotated between groups with no more than two applications of the same group applied during a single growing season (except for chlorothalonil which can be applied up to 3 times with no restrictions in terms of application order). There is a high risk that the pathogen may develop resistance to fungicides with repeated use of the same fungicides.

Optimize the number and sequence of foliar fungicide sprays

The number of fungicide sprays during the season depends on chickpea variety and disease severity. With more resistant varieties, such as CDC Frontier, one to two sprays may be sufficient in a normal season. However, if above-average rainfall occurs during flowering, more than two sprays may be needed. Highly susceptible varieties, such as CDC Xena and Sanford, may require up to 6 sprays. In paired-row plantings, the rate of fungicides can be reduced by 1/3 because of improved coverage with the 3-nozzle sprayer system (Fig. 4b). Fungicide sprays are no longer necessary when the crop reaches the late-pod stage, regardless of disease severity at that time. If applied after late-pod stage, fungicide may delay plant maturation.

Optimize spray application timing

Timing of first spray is the key to the success of any spray program. It is critical to apply the first foliar fungicide for ascochyta blight before flowering. Air-borne spores are present right from the beginning of the growing season. A preventative fungicide application during seedling stage is warranted to delay the onset of disease. This should be done regardless of the presence of moisture or blight symptoms on the plants. Timing of any consecutive sprays can be determined based on rainfall and blight symptoms.

The number of plant nodes can be used to time the first spray. After plant emergence, the number of nodes on the stems increases progressively with rising temperatures. Under normal growing conditions, the plant starts flowering at the 12 to 14 node stage (Fig. 5). The ideal timing of first spray is recommended to be between the 8th and 10th node stage (about a week before flowering).

Description of this image follows.

Fig. 5. Progress of node development on main chickpea stem. The initial spray is recommended when number of nodes is below the green band.

Description - Figure 5

Changes in the timing of the beginning of flowering and the pattern of increasing number of nodes on the chickpea main stem over the plant growth period from 0-80 days after planting in chickpea crops seeded in mid-May and early-May. Application of the first foliar fungicide is recommended before flowering starts or when the number of nodes is below the intersection of a horizontal line linking the beginning of the flowering (12-14 nodes) with the node number on the vertical axis (shown with the green band).

Days after initial plant emergence Number of nodes for early-May seeded chickpea Number of nodes for mid-May seeded chickpea
0    
10 4 4
20 6 10
30 9 13
40 13 18
50 18 22
60 19 24
70 20 25

Use an appropriate sprayer system

It is essential to use a sprayer system that is adapted to deliver the product to the central part of the crop canopy that is most vulnerable to disease. The modified 2-armed, 3-nozzles system (Fig. 4B) to has been shown to achieve optimal coverage of foliage due to a superior penetration of the spray droplets into plant canopy, especially in a paired-row seeding setting. This system has great potential to reduce fungicide rates through minimizing the loss of fungicides between the rows in an open canopy and improving spray efficiency. In addition, proper carrier volume is important for adequate coverage, with a minimum volume of 100 L per hectare (40 liters per acre) being required. Higher carrier volume improves coverage and enhances fungicide efficacy.

III. How to choose the best control package?

Integrating a variety of tactics described in this guide is the best approach to achieve good results in minimizing the devastating effects of ascochyta blight in chickpea. Integrated disease control systems require a combination of available cultural practices along with genetic resistance to keep plant disease below economically damaging levels. Use of fungicides is part of the integrated approach to ensure reliable production, especially for susceptible varieties. However, using an integrated approach will reduce the overall dependency on chemicals, reduce input costs and decrease the potential impact of pesticides on the environment.

Control tactics described above may perform differently in crop varieties that vary in the level of susceptibility. Therefore, growers are encouraged to develop and adopt customized management packages that are best suited to their cropping and field conditions. Work is underway to develop more detailed ascochyta blight management packages for existing chickpea varieties.

IV. Testimonials

Photo: Troy LaForge, manager of a leading consulting firm at Swift Current (shown here on the right with a Research Scientist from Australia, at a chickpea field of southern Saskatchewan)

'With the research results from Yantai Gan and his team we now understand proper stubble to seed into, proper plant population and more effective disease control programs. This information has helped to improve plant maturity, reduce disease severity and increase yield and quality for my clients and other producers,' says Troy LaForge, manager of a leading consulting firm at Swift Current (shown here on the right with a Research Scientist from Australia, at a chickpea field of southern Saskatchewan).

Photo: John Bennett, Bigger, Saskatchewan

'There is no 'Magic Bullet' to successful farming. Successes come from the integration of multitudes of strategies. Dr. Gan's research has been helping my farm to improve the bottom lines by addressing most of the variables in the production systems. Dr. Gan's systems approach optimizes many phases of the production cycles, starting right from seed selection and treatment, seeding configuration, crop management, all the way to harvest. Over 20+ years of growing pulses on my farm, we have learned a great deal from many mistakes and unknowns. This paper flags most of these problems and recommends excellent methods of addressing them.' says John Bennett, farming at Bigger area, Saskatchewan.

Disclaimer

This publication was prepared by Dr. Yantai Gan and, for the most part, is based on the results of his research in recent years. The content has been reviewed by other experts in the field of chickpea pathology and integrated disease management. This publication is meant to serve as a source of information on disease control techniques. Provincial crop production guides and fungicide labels should be consulted for information on specific uses and precautions. Disease management decisions are the responsibility of individual growers.

Acknowledgment

The Pest Management Centre acknowledges the contribution and support of all collaborators in conducting the study and reviewing this publication, especially Dr. Bruce Gossen, Dr. Tom Wolf, Cal McDonald, Lee Poppy, Greg Ford, and Ray Leshures of AAFC; Drs. Tom Warkentin and Sabine Banniza of Crop Development Centre, University of Saskatchewan; Penny Pearse, Faye Dokken and Ray McVicar of Saskatchewan Ministry of Agriculture.

For more information

Contact Dr. Yantai Gan
Agriculture and Agri-Food Canada
Swift Current, Saskatchewan
yan.gan@agr.gc.ca
Phone: 306-778-7246

Or consult relevant on-line publications such as:

Varieties of Grain Crops 2009

Field Scouting Tips for Ascochyta Blight of Chickpea

Pulse Crop Diseases


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